1. Field of the Invention
This invention relates to a turret bearing structure for vessels such as offshore drilling or production vessels, and more particularly to such a turret bearing structure having bearing pads for reacting horizontal loads between the turret structure and the vessel.
2. Description of the Prior Art
Heretofore, vessel mooring systems have been provided in which a vessel may weathervane about a turret which is normally anchored to the sea floor. The turret extends through a moon pool or cylindrical opening that extends through the hull of the vessel. The vessel is supported for rotation with respect to the turret by bearing structures placed between the turret and the vessel. Horizontal and vertical bearings transfer horizontal and vertical loads between the turret and the vessel.
U.S. Pat. No. 5,746,148, to Delago issued May 5, 1998, shows a radial roller assembly which provides radial support of a rotatable vessel to a fixed cylindrical turret in order to resist horizontal forces. The radial roller assembly includes wheels that are arranged to contact a rail provided around the turret and that are arranged to roll about the rail depending on the position of the rotating vessel relative to the fixed turret. The wheels are each arranged in an individual support carriage that includes a spring for absorbing the horizontal load placed on the wheels by the vessel.
U.S. Pat. No. 5,860,382 to Hobdy issued Jan. 19, 1999 shows a radial bearing arrangement for radially supporting a vessel with respect to a substantially vertically aligned cylindrical turret which is rotatably mounted within a vertical opening or well of the vessel. The radial bearing arrangement includes a rail secured to the outer circumference of the turret and plurality of radial bearing assemblies mounted on the structure of the vessel and spread at arcuate intervals about the outer circumference of the turret for engaging the rail. Each radial bearing assembly includes a roller rotatably mounted thereon for bearing contact against the bearing rail on the turret. A pair of concentric spring assembly continuously urge a follower against the roller frame into riding contact with the turret rail.
One shortcoming of the prior art is that the number of radial roller assemblies that can be used to absorb horizontal loads is limited by the geometry of the vessel well that surrounds the turret. That is, the well has a fixed diameter, and the number of radial roller assemblies that can be placed at the well is limited to the number that can fit around that diameter. This means that the radial load capacity of the roller assemblies is limited. This limitation may be a problem if the horizontal loads exerted between the vessel and the turret exceed the radial load capacity of the roller assemblies.
3. Identification of Objects of the Invention
It is an object of the invention to overcome the disadvantages of the prior art.
Another object of the invention is to provide a radial bearing arrangement having bearing pads arranged to absorb radial loads.
Another object of the invention is to provide a radial bearing arrangement that has a secondary stop to assist a radial spring assembly in limiting radial movement of a vessel relative to a turret.
Another object of the invention is to provide a radial bearing arrangement with fewer radial rollers for the same load capacity as existing designs.
Another object of the invention is to provide a radial bearing arrangement between a vessel and turret which has an increased radial load capacity for the same number of radial rollers as existing designs.